1. Field of the Invention
This invention relates to the process for treating metal surfaces to become more corrosion resistant, and more particularly to an improved method for chromating a metal selected from the group consisting of zinc, cadmium and alloys thereof.
2. Description of the Prior Art
Surfaces of zinc, cadmium and alloys have become more corrosion resistant when treated in a solution containing chromium ions. Depending on the content of such solutions, blue, yellow, black or olive-colored surfaces are created which not only have marked differences in color, but also in corrosion resistance. Especially blue chromates yield light blue surfaces with good corrosion resistance.
A measure of corrosion resistance of such chromating solutions is a salt spray test according to ASTM B-117 protocol wherein a plated and passivated part is subjected to a 5% salt spray at 35.degree. C. and 100% humidity. The time is measured to oxidize 5-10% of the surface to white rust. An alternative method has been developed by the EG&G Princeton Applied Research (Potentiostat/Galvanostat) may also be utilized in conjunction with software and an Apple II computer to measure corrosion resistance wherein the sample is immersed in an electrolyte and the current-potential-curve around the equilibrium potential is measured against a platinum electrode with the aid of a reference electrode. The resultant curves permit the calculation of the loss of metal in mpy (mils per year) with the smaller value being a better measure of corrosion resistance. The conducting electrode is comprised of 50 g/l NaCl, 10 ml/l phosphoric acid (85%) adjusted to pH 6.0 with NaOH. The second protocol has the advantage over the ASTM B-117 protocol in that corrosion resistance is provided in a short time, for example, about 15 minutes.
In U.S. Pat. No. 4,263,058/DE-OS 30 38 699 there is described an acidic, chrome-containing passivating solution comprising in addition to a "blue solution" of trivalent chromium ions, an acid, such as formic-, acetic-, propionic-acid or nitric-, sulfuric-, hydrochloric-, and hydrofluoric-acid. Also present are fluorides which form a "green solution" from hexavalent chromium ions, for instance, chrome-trioxide, alkali-metal-chromate of di-chromate and a reducing agent, such as an aldehyde or alcohol or an alkali-metal-sulfite, -bisulfite, -metabisulfite, -iodide, hydrogen peroxide, sulfur dioxide or an iron-II-salt-II-salt.
The presence of the poisonous hexavalent chromium compounds in the chromating solution permits the buildup of the hexavalent chromium into the surface of the passivating film, which can lead to dermatological illness if such films are used in the food processing industry. Handling of such solutions in the plant as well as waste treatment of used solution or rinse water can cause problems. Additionally, such solutions are rapidly consumed and can be regenerated only on a limited scale with concomitant high costs for continuous waste treatment and new make-up.
For several years, attempts have been made to produce blue chromates by using the tervalent chromium ion in the absence of the hexavalent chromium ion. Acidic solutions of such materials can also contain additional oxidizing agents (U.S. Pat. No. 4,171,231). Other additives such as silicates and/or metallic ions (U.S. Pat. No. 4,384,902, 4,389,247 and 4,367,099 ) or organo phosphorus compounds (U.S. Pat. No. 4,539,348) or carbonic acid (U.S. Pat. 4,349,392) disclose other additives. Such disclosed processes produce decorative blue- or yellow-chromated surfaces which cannot be subsequently dyed, with corrosion protection values reaching 6 hr. to 10% white rust according to the ASTM B-117 protocol. The presence of such oxidizing agent cannot assure the absence of hexavalent chrome compounds in the thus produced coatings. Still further, the waste water treatment is much more difficult in the presence of an oxidizing agent as a result of a rise in pH values in the sedimentation tank.
In German Patent 38 12 076, there is disclosed a process for using tervalent chrome compounds in fixed amounts together with soluble nitrates, fluoride and hydrochloric or nitric acid when the mixture is either heated to 60.degree. C. or treated with a catalyst, like activated carbon, above 15.degree. C. during which there is formed the green chrome-fluoride-complex of the general formula; EQU [Cr(H.sub.2 O).sub.6 -xF.sub.x ].sup.(3-x).spsp.+ with 0&lt;x=&lt;3
Such a passivating solution is very sensitive to iron contamination since such acidic chromating solution dissolves small amounts of iron from plated steel parts, especially from such parts which, because of their geometrical shape, are not totally covered with zinc. Additionally, such protocol requires constant pH control since the process can be only operated successfully over a narrow pH range of from 1.6-2.0. The existence of fluoride, however, destroys the electrodes making it impossible to automatically control the pH. Additionally, the removal of fluoride from the effluent is difficult with high concentrations of make-up required with concomitant high costs as well as high drag-out losses.